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# Introduction to Structural Member Properties

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Introduction to Structural Member Properties
Moment of Inertia Principles of EngineeringTM Unit 4 – Lesson Statics Introduction to Structural Member Properties

Structural Member Properties
Moment of Inertia Principles of EngineeringTM Unit 4 – Lesson Statics Moment of Inertia (I) is a mathematical property of a cross section (measured in inches4) that gives important information about how that cross-sectional area is distributed about a centroidal axis. Stiffness of an object related to its shape In general, a higher moment of inertia produces a greater resistance to deformation. ©iStockphoto.com ©iStockphoto.com

Moment of Inertia Principles
Principles of EngineeringTM Unit 4 – Lesson Statics Joist Plank Because of the orientation, the joist has a greater moment of inertia. The joist is 9 times as stiff as the plank in this example. Beam Material Length Width Height Area A Douglas Fir 8 ft 1 ½ in. 5 ½ in. 8 ¼ in.2 B

Moment of Inertia Principles
Principles of EngineeringTM Unit 4 – Lesson Statics What distinguishes beam A from beam B? Because of the orientation, the joist has a greater moment of inertia. The joist is 13.4 times as stiff as the plank in this example. Will beam A or beam B have a greater resistance to bending, resulting in the least amount of deformation, if an identical load is applied to both beams at the same location?

Calculating Moment of Inertia – Rectangles
Moment of Inertia Principles Moment of Inertia Principles of EngineeringTM Unit 4 – Lesson Statics Why did beam B have greater deformation than beam A? Difference in moment of inertia due to the orientation of the beam Calculating Moment of Inertia – Rectangles h

Calculating Moment of Inertia
Principles of EngineeringTM Unit 4 – Lesson Statics Calculate beam A moment of inertia

Calculating Moment of Inertia
Principles of EngineeringTM Unit 4 – Lesson Statics Calculate beam B moment of inertia

Moment of Inertia 14Times Stiffer Beam A Beam B Moment of Inertia
Principles of EngineeringTM Unit 4 – Lesson Statics 14Times Stiffer Beam A Beam B

Moment of Inertia – Composite Shapes
Principles of EngineeringTM Unit 4 – Lesson Statics Moment of Inertia – Composite Shapes Why are composite shapes used in structural design?

Non-Composite vs. Composite Beams
Moment of Inertia Principles of EngineeringTM Unit 4 – Lesson Statics Doing more with less Calculating the moment of inertia for a composite shape, such as an I-beam, is beyond the scope of this presentation. The value is given for purposes of comparison. Both of these shapes are 2 in. wide x 4 in. tall, and both beams are comprised of the same material. The I-beam’s flanges and web are 0.38 in. thick. The moment of inertia for the rectangular beam is in.4 Its area is 8 in.2. The moment of inertia for the I-Beam is 6.08 in.4. Its area is 2.75 in.2. The I-beam may be 43% less stiff than the rectangular beam, BUT it uses 66% less material. Increasing the height of the I-beam by about 1 inch will make the moment of inertia for both of the shapes equal, but the I-beam will still use less material (61% less). Area = 8.00in.2 Area = 2.70in.2

Structural Member Properties
Moment of Inertia Principles of EngineeringTM Unit 4 – Lesson Statics Structural Member Properties Chemical Makeup Modulus of Elasticity (E) The ratio of the increment of some specified form of stress to the increment of some specified form of strain. Also known as coefficient of elasticity, elasticity modulus, elastic modulus. This defines the stiffness of an object related to material chemical properties. In general, a higher modulus of elasticity produces a greater resistance to deformation.

Modulus of Elasticity Principles
Moment of Inertia Modulus of Elasticity Principles Principles of EngineeringTM Unit 4 – Lesson Statics Beam Material Length Width Height Area I A Douglas Fir 8 ft 1 ½ in. 5 ½ in. 8 ¼ in.2 20.8 in.4 B ABS plastic

Modulus of Elasticity Principles
Moment of Inertia Principles of EngineeringTM Unit 4 – Lesson Statics What distinguishes beam A from beam B? Will beam A or beam B have a greater resistance to bending, resulting in the least amount of deformation, if an identical load is applied to both beams at the same location?

Modulus of Elasticity Principles
Moment of Inertia Principles of EngineeringTM Unit 4 – Lesson Statics Why did beam B have greater deformation than beam A? Difference in material modulus of elasticity – The ability of a material to deform and return to its original shape Characteristics of objects that affect deflection (ΔMAX) Applied force or load Length of span between supports Modulus of elasticity Moment of inertia

Calculating Beam Deflection
Moment of Inertia Calculating Beam Deflection Principles of EngineeringTM Unit 4 – Lesson Statics Beam Material Length (L) Moment of Inertia (I) Modulus of Elasticity (E) Force (F) A Douglas Fir 8.0 ft 20.80 in.4 1,800,000 psi 250 lbf B ABS Plastic 419,000 psi

Calculating Beam Deflection
Moment of Inertia Calculating Beam Deflection Principles of EngineeringTM Unit 4 – Lesson Statics Calculate beam deflection for beam A Beam Material Length I E Load A Douglas Fir 8.0 ft 20.80 in.4 1,800,000 psi 250 lbf

Calculating Beam Deflection
Moment of Inertia Calculating Beam Deflection Principles of EngineeringTM Unit 4 – Lesson Statics Calculate beam deflection for beam B Beam Material Length I E Load B ABS Plastic 8.0 ft 20.80 in.4 419,000 psi 250 lbf

Douglas Fir vs. ABS Plastic
Moment of Inertia Douglas Fir vs. ABS Plastic Principles of EngineeringTM Unit 4 – Lesson Statics 4.24 times less deflection

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